Patent GB-343392-A (GB)
Abstract: 343,392. Multiplex wireless signalling. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., Marconi House, Strand, London.- (Assignees of Finch, J. L.; Rocky Point, Long Island, New York, U.S.A.) April 25, 1930, No. 12847. Convention date, May 31, 1929. [Classes 40 (iii) and 40 (v).] In a multiplex signalling system, energy of constant frequency and amplitude is transmitted intermittently for periods having a definite relation to the amplitude of the complex wave resulting from the combination of a number of signalling waves. The broad principle of the system is shown in Figs. 3 and 4, in which the complex wave 8 is the resultant of three signalling waves 2,4, 6, the transmitting aerial being excited for periods having an inverse relation to the amplitude of wave 8. By the provision of a suitable rectifier and filter at the receiving end, complex curve 8 can be simulated when receiving energy transmitted in accordance with Fig. 4, the signal waves 2, 4, 6 being separated by suitable filters. The transmitting arrangements are shown in Fig. 1, in which signal frequencies from the tone generators 21, 4<1>, 61 are combined with a special form of carrier, generated at B, and impressed on the input of valve 36. Circuit B comprises a crystal rectifier 20 connected across a battery 22 through a choke 26 and resistance 24, a condenser 28 being shunted across the rectifier and choke. The arrangement is such as to produce a wave form as shown in Fig. 5 the frequency being several times higher than the highest tone frequency from A but low compared with radio-frequency currents. The grid of valve 36 is negatively biased so that little current flows through resistance 44, in parallel with which is a glow discharge tube 46 and resistance 48. The tube 46 passes current when a certain potential is applied across it, the consequent voltage drop across 48 acting as a negative bias to the grid of valve 52. The potential between tappings uv on the battery 40 is added to that impressed across the resistance 44 and causes tube 46 to discharge with a lower total voltage. When the grid of valve 52 is negative, current through resistance 54 decreases and the grid of valve 56 becomes slightly positive so that a large current flows through resistance 58. As this resistance is in series with the anode voltage supply to the valve 62, which is an amplifier for the radiofrequency carrier generated at 60, the potential drop produced across 58 will oppose the voltage from 68 and the amplifier 62 will be rendered inoperative. A voltage wave of the form shown in Fig. 6 is applied to the input of valve 36 and owing to the negative bias on this valve its anode current and therefore the voltage across resistance 44 will take the form shown in Fig. 7. Added to this is the voltage uv which is slightly less than the breakdown voltage Eb of the tube 46, so that current flows through resistance 48 during the periods OP, O<1>P<1>, &c. of Fig. 7. These peaks of current produce potentials across resistance 58 as shown in Fig. 8, resulting in an aerial excitationtime curve as in Fig. 9. The receiver, Fig. 2, (not shown), comprises an amplifier, detector, and filter to remove the carrier frequency from B, Fig. 1, followed by separate filters to separate the various signal tcne frequencies.